Many gravimetric measuring instruments have a parallel-guiding mechanism. A parallel-guiding mechanism includes at least one stationary and one movable parallelogram leg as well as at least one upper and one lower parallel-guiding member. A weighing pan which is supported and guided in vertical movement by the movable parallelogram leg receives the weighing load, wherein the force exerted by the load on the weighing pan is transmitted either directly or through a force-reducing lever mechanism to a measurement transducer. The parallel-guiding mechanism, the force-transmitting system and the measurement transducer constitute in essence the weighing cell of a gravimetric measuring instrument. The current state of the art includes a variety of operating principles of weighing cells such as for example weighing cells with strain gauges, weighing cells with oscillating strings, or weighing cells with electromagnetic force compensation (EMFC).
In EMFC weighing cells, the weight of the load is transmitted either directly or through one or more force-transmitting levers to an electromechanical measurement transducer which produces an electrical signal that is representative of the weighing load and is further processed and visually displayed by an electronic weight-processing system.
In weighing cells with strain transducers, an elastically deformable body is equipped with strain gauges. The deformable body changes its shape elastically under the applied load. The deformable body is in many cases configured as a parallelogram-shaped measuring element, specifically as a parallel-guiding mechanism with specially shaped bending zones, whereby defined deformation zones are established. The strain gauges are arranged in the areas of these deformation zones or bending zones. As a result of the shape change of the deformable body due to a load on the movable parallelogram leg, the strain gauges are subjected to a state of tension or compression, which causes a change of the electrical resistance of the strain gauges in comparison to a load-free state of the movable parallelogram leg, wherein the resistance change represents a measure for the magnitude of the applied load.
In string-oscillator weighing cells, the overall mechanical configuration is largely analogous to the EMFC- and strain gauge weighing cell, with the difference that an oscillating string transducer is used in place of the electromagnetic transducer. The weighing load affects the tensile force in an oscillating string whose frequency change, in turn, represents a measure for the applied load.
One characteristic trait of the weighing cells of the foregoing description, which all gravimetric measuring instruments with a weighing pan constrained by a parallel-guiding mechanism have in common, is the property that the weight force transmitted from the weighing pan to the transducer generally shows a slight dependency on whether the weighing load is placed on the center of the weighing pan or off-centered towards the edge. This can have the undesirable consequence that a gravimetric measuring instrument, i.e. a balance, indicates a different amount of weight for one and the same weighing load, depending on where the load was placed on the weighing pan. These deviations which occur when the weighing load is placed eccentrically on the weighing pan are commonly called eccentric load errors (or also as shift errors or corner load errors).
In a parallelogram-shaped measuring element or parallel-guiding mechanism, i.e. a mechanism which guides the weighing pan support in a parallel movement by means of two parallel-guiding members which are parallel to each other and essentially horizontal, eccentric load errors occur primarily due to the fact that the parallel-guiding members deviate slightly from the ideal, absolutely parallel alignment. The relative magnitude of the eccentric load error, i.e. the ratio between the observed weighing error and the size of the test weight being used, approximately corresponds to the relative geometrical deviation which caused the eccentric load error. A distinction is made between an eccentric load error in the lengthwise direction and an eccentric load error in the transverse direction of the parallel-guiding mechanism, in accordance with the direction in which the test weight is moved on the weighing pan in an eccentric load test of the balance. An eccentric load error in the lengthwise direction occurs if the vertical distance between the parallel-guiding members at the end which is connected to the stationary parallelogram leg is not exactly equal to the distance at the opposite end which is connected to the movable parallelogram leg. An eccentric load error in the transverse direction, on the other hand, occurs if the two parallel-guiding members are slightly twisted relative to each other, i.e. if the distance between the parallel-guiding members varies over the width of the parallel-guiding members.
In state-of-the-art references, for example U.S. Pat. No. 6,326,562, which is commonly-owned, as well as in JP 2002 365125 A and WO 2005/031286, parallel-guiding mechanisms of weighing cells are disclosed which include a device for adjusting the eccentric load error. The adjustment mechanism in these weighing cells follows a design concept in which the stationary parallelogram leg has at least one deformation zone between the areas of attachment of the parallel-guiding members, wherein the deformation zone is configured in such a way that a tilt axis is defined whose orientation is orthogonal to the lengthwise direction of the parallel-guiding mechanism. By tilting the areas of attachment against each other by means of an adjustment screw, the end of the upper parallel-guiding member that is connected to the stationary parallelogram leg can be raised as well as lowered. This allows the eccentric load error in the lengthwise direction to be corrected. Depending on the design of the adjustment mechanism, the inclination angle of the tilt axis, i.e. the transverse inclination of the area of attachment, could likewise be adjustable, which would allow the eccentric load errors to be adjusted for the transverse direction of the weighing cell.
Eccentric load error adjustments can be made as long as the parallel-guiding mechanism of the weighing cell is freely accessible. However, if the weighing cell is enclosed in an encapsulation which cannot be reopened, as disclosed in commonly-owned U.S. Pat. Nos. 5,895,894 or 4,957,177, an eccentric load error adjustment is no longer possible. As an additional problem, which is described in detail in U.S. Pat. No. 4,957,177, the encapsulation itself in the form of a bellows can have an influence on the eccentric load accuracy or eccentric load error. As a result of this, mechanically identical encapsulated weighing cells will differ from each other in their eccentric load errors even if their parallel-guiding mechanisms were adjusted prior to installing the encapsulation. The only possibility left is to compensate for these variations through the expensive measure of a signal-processing unit connected to the transducer.
Consequently, the present disclosed embodiments has the objective to create a gravimetric measuring instrument with an encapsulated weighing cell which is free of the drawbacks regarding the eccentric load accuracy which have been described above.